This invention relates to sealing means, and in particular, to seals employing a gas lift bearing to prevent intermixing of fluids in rotary machinery.
Where rotary members such as shafts are employed, it is often necessary or desirable to provide some means to prevent the passage or leakage of fluids along the shaft. Specific sealing applications often require that fluids contacting one part of the rotary shaft be prevented from coming into contact with or intermixing with fluids in contact with another portion of the shaft. Conventional sealing members have been provided in various arrangements in attempting to overcome the problems associated with changing pressure differentials due to start up, shut down, load change, etc., as well as fluid contamination. Since carbon seals are subject to wear and burning when run at high tip speeds, they have been limited in application and are not generally suitable to prevent fluids from contacting or intermixing in high speed applications.
In binary geothermal applications, for example, the geothermal fluid exhanges heat with a hydrocarbon fluid, thereby causing it to be vaporized. The hydrocarbon vapor is then expanded through a turbine to develop power. Because of the explosive nature of hydrocarbon vapors, it is necessary to minimize the leakage of this gas to atmosphere and to assure the long term reliability of the seals. This is also true for centrifugal compressors which compress explosive or toxic gases to high pressure.
Previously, carbon ring seals were ground to light-band tolerances to assure parallelism between the carbon ring and shaft runner. This prevented vibration and wobble of the seal during operation. Carbon rings, because of the nature of carbon, provide some lubricity between the mating running surfaces during start up; however, they rely on the migration of oil toward the gas during operation, to provide a frictionless running surface. This oil is eventually collected in a contaminated drain and disposed of.
As pitch line speeds (proportional to the product of seal diameter and RPM) increase, the heat generated between the mating surfaces can no longer be controlled by the oil migration, thus scoring and wear of the carbon ring occurs, since separation of the mating carbon ring and shaft runner cannot be maintained.
Additionally, the light-band tolerance on carbon seals is difficult to achieve and maintain. If high spots on the seal occur, there will be excessive heat generated, resulting in wear and scoring of the seal surfaces.